Problem: $-8st + 3su + s + 9 = 6t + 4$ Solve for $s$.
Answer: Combine constant terms on the right. $-8st + 3su + s + {9} = 6t + {4}$ $-8st + 3su + s = 6t - {5}$ Notice that all the terms on the left-hand side of the equation have $s$ in them. $-8{s}t + 3{s}u + 1{s} = 6t - 5$ Factor out the $s$ ${s} \cdot \left( -8t + 3u + 1 \right) = 6t - 5$ Isolate the $s$ $s \cdot \left( -{8t + 3u + 1} \right) = 6t - 5$ $s = \dfrac{ 6t - 5 }{ -{8t + 3u + 1} }$